Our long term goal is to understand how sex steroids regulate G protein-coupled receptor mediated neurotransmission that underlies hormonal activation of behavior. We have demonstrated that membrane-initiated estradiol signaling (MIES) activates a circuit regulating lordosis behavior, a measure of sexual receptivity. Estradiol sequentially activates estrogen receptor-a (ERa), releasing NPY onto NPY-Y1 receptors in the ARH, which stimulates -endorphin release into the medial preoptic nucleus activating -opioid receptors (MORs). The present proposal builds on our discovery that MIES requires the metabotropic glutamate receptor, mGluR1a, to initiate cell signaling and that estradiol modulates membrane levels of ERa. We propose to test the hypothesis that MIES, which allows for hormonal-neural integration, regulates ERa trafficking to and from the membrane, and spinogenesis needed for sexual receptivity. Three Specific Aims are proposed: 1) MIES modulates ERa-mGluR1a levels in the membrane which will be tested through a series of pharmacological studies that systematically block or activate MIES and intracellular pathways. 2) Lordosis behavior depends on MIES-induced trafficking of ERa-mGluR1a to the membrane. Manipulations that alter ERa-mGluR1a trafficking in the ARH will be tested on MOR internalization, a measure of circuit activation, and lordosis behavior. 3) MIES integrates hormonal and neuronal signals inducing morphological reorganization of the ARH underlying sexual receptivity. We will determine how estradiol, by activating different intracellular signaling pathways, induces dendritic spine formation and maturation needed for lordosis behavior. These experiments have salience for understanding not only reproductive behavior, but estradiol cell signaling throughout the nervous system through their interaction with mGluRs.